Custom wood connection at a concrete wall.
Certain top flange-mounted hangers can be welded directly to steel beams. This can be advantageous when a deep steel beam is required in a limited floor-to-floor height or depth-limited ceiling space. However, for plywood sheathed floors, consideration must be given to how the edges of the plywood diaphragm will attach to a steel beam to transfer diaphragm force if required. This is especially important where the steel beam is in place to transfer a discontinuous shear wall above. Powder-actuated fasteners, carriage bolts, or sex bolts are options. Top flange mount hanger welds to steel beams are typically ⅛- to 3⁄16-inch fillet welds with minimum lengths specified by the manufacturer. Wood blocking is not typically required behind the welded joist hanger. However, web stiffeners may be required for the steel beam based on loading configuration, the loading magnitude, and steel flange and web thicknesses. Depending on the type of construction, welder quality can vary widely. Therefore, engineers should consider both the quality of welding available and the additional labor/trade required when specifying top flange mounted, welded hangers.
Wood-to-Steel Column Connections When steel beams are introduced to a building in the floor framing, it frequently results in steel columns being introduced as well. For this discussion, the authors are not including lally columns used in traditional basement construction. Steel columns can be wide flange sections, but they are frequently hollow structural sections (HSS) to fit within wood stud framed walls. Sometimes, wood elements connect to steel columns by prefabricated welded joist hangers. Depending on the column flange width, welded joist hangers attach directly to a base steel member and can have concealed or extended flanges. For HSS columns, the joist hanger width should be coordinated with the workable flat-tube dimension and not the nominal dimension. The welds are typically 1 ⁄16-inch fillet or flare bevel groove stitch welds, symmetrically placed. The small weld size prevents burn-through of the metal hangers; as noted above, this requires skilled, qualified welders. Therefore, even for small projects, specifying an AWS-certified welder to perform these welds is valuable. Not every joist hanger can be welded, so check the manufacturer’s product literature before specifying a welded joist hanger. The manufacturer’s product literature will also provide the tested weld sizes and weld configurations along with their allowable load carrying 38 STRUCTURE magazine
capacities. The load-carrying capacity is typically the lesser value between the joist hanger capacity and the weld capacity. In seismic regions, consideration should be given to lateral forces at these hangers since manufacturers typically do not publish allowable lateral values for weldable joist hangers. As an alternative, custom wood-to-steel connections can also be designed and utilized for situations where load demand, connection geometry, or architectural requirements preclude the use of readily available prefabricated joist hangers. In this case, the options are endless. However, the downside to these connections is the high material and labor costs and engineering time relative to traditional joist hangers. Therefore, custom hangers should be designed by the structural engineer and not delegated to the contractor. For all welded connections, base metal preparation is key to a successful connection. Base steel that has surface rust should be cleaned before welding. Welding over paint is discouraged by welding codes and can cause unsightly blemishes, release toxic fumes, and compromise the quality of the weld. Steel that has been primed requires cleaning prior to welding. Engineers should recognize the potential for less sophisticated welders in residential construction and be clear in welding requirements. Consider requiring weld-specific inspections by qualified inspectors for critical welds on a project.
Wood-to-Concrete Connections Designers must pay special attention to moisture, where wood is attached to concrete elements, typically at foundation walls and footings. Wood sill plates and rim boards attached to concrete foundation walls are typically pressure treated to protect against moisture migration from the concrete. The coordination of concrete anchors, post bases, steel embed plates, beam pockets, and ledges is also important. Locating these elements before concrete foundation placement requires up-front coordination but saves time and cost overall for the project. If the contractor prefers to post-install these anchors, they should scan for reinforcement before drilling. Wood nailers and plates are typically anchored to concrete through cast-in threaded rods or J-bolts. They are often doubled up to provide adequate nailing for plywood-sheathed shear walls and diaphragms. Where double sill plates or rim boards are used with cast-in anchors, contractors must coordinate the anchor’s location and extension beyond the concrete surface to ensure adequate thread length for the nuts.
Summary As manufactured wood product options continue to increase, architects, contractors, and engineers should understand the advantages and disadvantages of each system, material, and product option. There are numerous configurations in which wood elements connect to other wood elements, structural steel, and concrete. Engineers can design with prefabricated and custom connectors and create ever-more complex and exciting structures. However, to do so successfully, engineers need to understand both the opportunities and limitations of each material product, the contractor’s experience with different systems, and the available trade people.■ All authors are with Simpson, Gumpertz & Heger in Chicago. Becky Havel is a Consulting Engineer. (rshavel@sgh.com) Rose McClure is a Senior Consulting Engineer. (rfmcclure@sgh.com) Matt Johnson is a Principal. (mhjohnson@sgh.com)
